Hydrocarbon pipelines, including those underwater, are built by joining pipe sections, each of which normally comprises a metal, normally steel, cylinder, to which are applied a protective polymer coating to protect the metal cylinder, and an optional outer covering of Gunite or cement to weigh the pipeline down.
The opposite free ends of each pipe are left bare to weld the steel cylinders to one another.
Joining he pipes, which may be carried out on land or (in the case of underwater pipelines) on laying vessels, comprises welding the steel cylinders, normally in a number of weld passes; and completing the protective polymer coating and the outer covering (if any). Once each two steel cylinders are welded, a bare annular joint portion (known as and hereinafter referred to as a “cutback”), defined substantially by the free ends of the pipes, extends astride the weld and axially between two end portions of the protective polymer coating, and must in turn be coated with a protective coating.
Applying the protective coating to the cutback is known as “Field Joint Coating”, and the cutback is normally coated with a number of coats of appropriate polymer material.
The most widely used methods normally apply three polymer coats:                a relatively thin first or primer coat applied directly on the cutback;        a relatively thin second coat of polymer adhesive, applied on top of the first coat; and        a relatively thick third or top coat (thicker, at any rate, than the first and second coats) applied on top of the adhesive coat.        
The outer covering, if there is one, is then also completed.
The two main methods currently adopted to apply three-coat protecting coatings of the above type are:
1) to apply the three coats separately, one after another;
2) to apply the first coat (primer), and then a heat-shrink sleeve comprising two layers corresponding to the second and third coats.
More specifically, the first method substantially comprises:                applying, e.g. spraying, the first coat (primer)—normally of powdered FBE (fusion bonded epoxy) resin—directly onto the cutback heated, e.g. induction heated, beforehand to a temperature of 200-250° C.;        applying, e.g. spraying, the second (adhesive) Coat—normally of polypropylene adhesive (modified propylene polymer or copolymer)—on top of the first coat; and        applying the third (top) coat of polypropylene (possibly modified) on top of the adhesive coat, e.g. using a hot spray gun capable of melting and applying the polymer, or by injecting the liquid polymer into a mold around the cutback.        
Other known ways of applying the third coat include:                so-called “cigarette wrapping”, whereby thin sheets of polymer material are heated, wrapped and compressed around the cutback, on top of the second coat; and        so-called “spiral wrapping”, whereby a strip of polymer material is heated, wound spirally and compressed around the cutback, on top of the second coat.        
The second method, employing heat-shrink sleeves, mainly differs from the first by simultaneously applying the second and third coats, incorporated in the heat-shrink sleeve.
The second method substantially comprises:                applying the first coat (primer)—in this case, of liquid epoxy (LE) resin;        fitting a heat-shrink sleeve (HSS) about the primed cutback; the sleeve normally comprises two layers: a protective, heat-shrink outer layer constituting the actual third coat; and an adhesive inner layer constituting the second (adhesive) coat; and        heating, e.g. flame heating, the sleeve to shrink the outer layer, melt the inner layer, and so bond the sleeve firmly to the first coat (primer) on the cutback.        
The above and other substantially similar methods of applying the protective coating leave room for improvement, especially in terms of easy, versatile, effective application, and performance of the finished coating, particularly in terms of mechanical strength and peeling.
Protective coatings formed using known methods, in fact, have proved far from satisfactory.
On the one hand, liquid epoxy (LE) resin primers have generally proved inferior to fusion bond epoxy (FBE) resin primers, which adhere better to the metal substrate, are more resistant to cathode detachment, and have good high-temperature stability and resistance.
On the other hand, commonly used FBE resins are poorly compatible, and therefore complicated to use, with currently available heat-shrink systems.
In other words, there is currently no versatile method, which can be used in various applications with various marketed systems, which is simple and effective, and which provides for fully satisfactory protective coatings.